vxlan: generalize forwarding tables

This patch generalizes VXLAN forwarding table entries allowing an administrator
to:
	1) specify multiple destinations for a given MAC
	2) specify alternate vni's in the VXLAN header
	3) specify alternate destination UDP ports
	4) use multicast MAC addresses as fdb lookup keys
	5) specify multicast destinations
	6) specify the outgoing interface for forwarded packets

The combination allows configuration of more complex topologies using VXLAN
encapsulation.

Changes since v1: rebase to 3.9.0-rc2

Signed-Off-By: David L Stevens <dlstevens@us.ibm.com>

Signed-off-by: David S. Miller <davem@davemloft.net>